Sometimes, in some places, the best thing the architect of a new building can do is to defer to another building -- even when the existing structure is hardly an architectural star.

The Cafritz mansion, perched on the crest of a hill on Foxhall Road, is one of those places. Combining the aesthetics of a French Renaissance chateau with those of an art moderne ocean liner, it's more a period curiosity than a weighty aesthetic treasure.

But the house has a fascinating history and, with its massive wings of strikingly white bricks, it does occupy that ridge with undeniable authority.

Thus, architects David Cox and Tom Wheeler of the Washington firm of Cox Graae + Spack did the right things by the old manse after getting the commission to convert the seven-acre Cafritz estate into a campus for an exclusive private school.

They made the house the linchpin of a convincing new architectural composition. They skillfully used the sloping terrain to disguise the bulk of the rather massive new additions. And they designed the new pieces in a crisply sympathetic vocabulary -- not a copy, mind you, but a comfortable fit. The result is a subtly satisfying transformation that looks almost effortless. Of course, it wasn't.

Real estate developer Morris Cafritz built the house in 1938, using Alvin Aubinoe and Harry Edwards, his company architects, to do the design. Presumably, Aubinoe and Edwards added the chateau touches to the house in deference to the tony conservatism of its Foxhall Road neighborhood, for the pair's apartment house designs were more consistently moderne.

In any case, the chief glories of the mansion were the splendid views of the capital city from the picture windows and terraces of its rear facade, and the moderne furnishings and interiors by New Yorker Eugene Schoen, perhaps the premier high-end interior designer of the day. Cafritz spared no expense and Schoen no effort in pursuing rare woods and fine finishes for the furniture and trimmings.

This house, in other words, was designed for entertaining on a lavish scale. It was the perch from which Cafritz's wife, Gwendolyn, a Hungarian beauty, could set her sights on Washington society. For the better part of four decades, it was the setting for glittery gatherings of the capital's power elite.

But by the late 1970s, the party era was definitively over. (Morris died in 1964, Gwendolyn in 1988.) Most of the furnishings were sold two years ago to benefit the Morris and Gwendolyn Cafritz Foundation, after a long dispute among Cafritz heirs was resolved. The house was purchased by the Field School, an innovative private academy for about 260 students in grades 7 through 12.

In a way, the mansion could hardly wish for a better caretaker than the school, whose founder and director, Elizabeth Ely, hates "long, Kafkaesque hallways" and loves small classrooms and an air of residential informality. Before the move, the school occupied two houses on opposite sides of Wyoming Avenue NW in Adams Morgan, and Ely says she wanted to preserve as much of that casual ambiance as possible in the upscale new digs.

To help achieve that aim, Ely and her board enlisted New York graphic artist Eric Kohler, who as a teenager had often visited the house with one of the Cafritz grandsons and who became, as an adult, an expert on Schoen furniture. Kohler brought in New York interior designer Daniel Sachs.

It was a complicated arrangement, creating tension between the New Yorkers and the Washington architects, but it worked. A good deal of the spirit of the original residence remains. The key rooms were kept in their original shapes; murals, floors and wall coverings were restored where possible, and some furnishings were replicated or, in the case of the streamlined dining room furniture, approximated.

Yet the house today unmistakably is a school. The old dance floor in a paneled basement room now supports chairs and tables for a study hall. Converted servant quarters easily fulfill Ely's desire for diminutive classes. In the living room, where politicians and Supreme Court justices used to sip vintage wines, students now gather for "United Nations simulations" in the company of the half-dressed figures in the half-serious allegorical mural above the fireplace.

The house itself, of course, is only part of the story -- a big part in symbolic terms but rather small in actual usage. Most of the classes, the gymnasium and a black-box theater are in the two new wings on either side of the mansion. Because of their size, these new facilities had the potential to overwhelm the original mansion but, thanks to the skill and sensitivity of Cox Graae + Spack, that didn't happen.

To the contrary, the new arrangement is extremely pleasing. Two crucial decisions during the design process paved the way for this result. The first was to separate the wings completely from the house. This choice entailed minor inconveniences -- students must go outside to get from building to building -- but in all other respects it gave the architects the freedom they needed.

Probably every architect on Earth carries an idealized view of an Italian hill town in his or her image bank. On this Washington hill, Cox and company clearly remembered -- by breaking the big buildings into discrete parts, they were able to give the ensemble a certain organic grace as it spills down the hill.

Furthermore, the effects are not simply picturesque. By giving the buildings such interesting shapes, the architects were able to create distinctive places on the grounds and thus to give the complex the genuine feel of a campus.

The L-shaped juncture of the gymnasium with a classroom wing, for instance, is both visually compelling and physically comforting. In contrast, the narrow, circular space between the house and the new main classroom structure is a dynamic gathering place.

Even inside the new buildings there are neat little nooks and crannies and other touches -- Kohler's abstract floor design outside Ely's office, for instance -- that give them a certain personality. Altogether, it adds up to thoughtful, humane architecture.

A second major decision involved architectural style. Initially, the architects proposed a self-consciously modern design for the additions, with lots of glass. But Ely didn't like that approach because, she says, it was insufficiently houselike.

In pushing the architects back to the drawing board, Ely was right, but maybe for the wrong reason. Though ostensibly more progressive, that initial design was nowhere near as compatible and compelling as the more modest, finely tuned second effort.

I cannot let this column go, however, without noting the painful comparison between the city's private and public schools. While the public schools go wanting, leaky roofs and all, Washington's architects are being kept busy by private institutions competing for wealthy parents with ever more splendid physical facilities. Cox, for example, reports that more than half of his business is now in private school work, and his isn't the only firm on this track.

On private campus master plans, what used to be called high school auditoriums are now referred to, for goodness sake, as performing arts centers. In the land of opportunity, some folks keep getting more and more -- including more architecture -- and some less and less.

The Cafritz mansion overlooking Foxhall Road holds its dominant place on what is now the private Field School campus, supported by the subtle compatibility of the two new buildings flanking it. Below, a closer view of the athletic complex.Sophomore Diana Ruskin has a captive audience as she practices in the mansion's former living room.